Ion beam measuring device



July 13, 1954 R. L. MATHER ION BEAM MEASURING DEVICE 2 Sheets-Sheet 1Filed Sept. 25, 1951 INVENTOR. ROBERT L. MATHER ATTORNEY.

July 13, 1954 MATHER 2,683,814

ION BEAM MEASURING DEVICE Filed Sept. 25, .1951 2 Sheets-Sheet 2 'sao3(0 340 320 so MEVI IN VEN TOR.

ROBERT L. MATHER awdmm A T TOR/V5 Y.

Patented July 13, 1954 UNITED STATES ATENT OFFICE ION BEAM MEASURINGDEVICE Application September 25, 1951, Serial N 0. 248,123

5 Claims. 1

This invention relates to an ion beam energy measuring device and moreparticularly to apparatus for determining the angle of emission ofsecondary radiation generated by bombardment as a function of the energyof the bombarding beam.

It has been demonstrated that electromagnetic radiation in the range ofvisible light is associated with the passage of a fast moving chargedparticle through matter. Such phenomenon has been explained by P. A.Cerenkov, whose name it bears, as an electromagnetic shock wave excitedby the passage of a charged particle through a medium with a velocitygreater than the velocity of electromagnetic radiation in the medium.Such shock Wave has a wavefront having the geometry of a cone with theapex located at the particle and extending symmetrically about the axislying along the path of the particle.

The foregoing phenomenon is utilized in the present invention bymeasuring the half angle of the cone as a function of beam energy. Toaccomplish such measurement it is necessary to accurately place a mirrorin angular relation with the beam to reflect the Cerenkov radiationthrough an optical system onto a suitable graduated scale.

It is therefore an object of the present invention to provide a new andimproved ion beam measuring device.

Another object of the present invention i to provide an ion beammeasuring device which reflects Cerenkov radiation onto a scale as afunction of beam energy.

Still another object of the present invention is to provide an accuratedetermination of the energy of a beam of particles from the Cerenkovradiation produced thereby.

A further object of the present invention is to provide an ion beammeasuring device wherein a mirror is accurately disposed in angularrelation with a beam of ions to reflect the Cerenkov radiation onto afilm.

Another object of the present invention is to provide an apparatus forforming Cerenkov radiation by a beam of charged particles anddetermining the angular relation of such radiation with the beam as ameasurement of the energy thereof.

A still further object of the invention is to provide an apparatus foraccurately measuring the energy of a beam of charged particles withoutsubstantially affecting the focus or energy of such beam.

Further objects and advantages will be apparent from the followingdescription and claims considered together with the accompanyingdrawing, in which:

Figure 1 is a sectional plan view of the present invention with the topplate removed;

Fig. 2 is an elevational view of the invention partly in section;

Fig. 3 is a sectional view taken along the line 3--3 in Fig. 2; and

Fig. 4 is a diagram illustrating a film of the projected Cerenkovradiation and a superimposed grid.

Referring to the drawing in detail, Figs. 1 and 2 in particular, thereis provided a platform having a pair of parallel and spaced-apartupright members |2 and |3 mounted thereon. Each of the upright membersl2 and I3 are accurately aligned with respect to one another and areprovided with V-cuts l6 and IT, respectively, in the uppermost portionsthereof.

A framework 2| having a base plate 22, two end plates 23 and 24, and atop plate 26 is provided with such plates suitably secured together, asby screws 21, to form a rigid boxlike structure having two opposing opensides. Each of the end plates 23 and 24 have hollow tubular trunions 2Band 29, respectively, mounted in alignment therein to provide acontinuous 1ongitudinal path therebetween. The longitudinal dimension ofthe base plate 22 and the top plate 26 is chosen to be such that theframework 2| may be rotatably positioned between the upright members l2and I3 with the trunnions 28 and 29 resting in the V-cuts l6 and I1. Toprevent longitudinal movement of the framework 2| in such position eachof the trunions 28 and 29 is provided with a collar 3| and 32,respectively, abutting against the inner surfaces of the upright membersI2 and I3.

For selectively positioning the framework 2| in four positions ninetydegrees apart, a square plate 38 having a slot 31 at each corner ismounted on the trunnion 28 and secured to the end plate 23, as by screws38, so that a diagonal lies in a plane parallel to the base plate 22. Toprovide a catch for locking the plate 36 in the desired position, a pawl4| is mounted on a rod d2 rotatabl extended between two aligned hearingblocks 43 and 44. Thus when the pawl 4| is rotated away from the plate36, the plate, and the framework 2 I, may be rotated to a new positionand then locked in such position by moving the pawl to engage in thealigned slot 3!.

A light-tight housing 5| is pivotally mounted within the framework 2|,as by opposing and aligned pivots 52 and 53 threadably extended throughtapped holes in the bottom and top plates 22 and 26, respectively, toengage bearings 56 and on the walls of the housing. To provide forpassage of a beam through the housing 5! a thin foil 58 permeable tosuch beam is mounted in an aperture, having substantially the samedimension as the hollow portions of the trunnions 28 and 29, disposed ineach of the opposing end walls of the housing in alignment with the pathbetween the trunnions. Preferably the center line of the pivots 52 and53 is disposed transversely to the axis of the path between thetrunnions 23 and 29 so that such center line and axis intersect. It isthus apparent that the angular position of the light-tight housing 5!may be adjusted with respect to the passage provided between thetrunnions 28 and 29. The necessity for such adjustment will be set forthhereinafter.

A circular plate GI, having a centrally disposed aperture $2 adapted toencircle the bearing 5?, is suitably secured to the housing 5! in theregion of the bearing by a plurality of screws 63 or other suitablefastening means. Two catch plates 65 and 5? are mounted against theunder surface of the top plate 26 of the framework 2| as by screws 58 toengage diametrically opposed circumferential portions of the circularplate 6 i A strip 69 of a resilient material having substantially thesame dimensions as the catch plates 88 and 6'! is secured between thecatch plates and the top plate 26 such that the circular plate Si isrotatably disposed between each catch plate and strip. Provided to serveas clamps are adjustable screws and i2 suitably threaded through the topplate to engage the strip 69 of each clamp plate and 51. Thus thelight-tight housing 51 may be positioned in a desired relation withrespect to the passage between the trunnions 23 and 29 and secured insuch position by means of the catch plates and 61 and the adjustablescrew H and E2.

Disposed within the light-tight housing 5! is a rectangular mirror ismounted on a holder Ti which is pivotally held in place by a supportingblock 38. The supporting block i8 is slotted and provided with atransversely extended screw 79 as a mean for clamping the holder i! in afixed position. Preferably the mirror 76 is mounted with its centerdisposed along the axis of the path between the trunnions 28 and 29 andthe longitudinal center line lying coincident with the axis of thealigned pivots 52 and 53. From the foregoing it is seen that the mirrormay be disposed angularly with respect to the path between the trunnion28 and 2t and fixed in such position by the screw '59. Also, after theangular position of the mirror 16 has been fixed, the light-tighthousing 5! may be rotated about the axis of the pivots 52 and 53 whenthe screws H and 12 have been loosened.

One of the side walls of the housing 5! is pierced by an aperture 8i toreceive a lens barrel 82 having a len 83 mounted therein. The internallyprojected portion of the barrel B2 is provided with threads for engagingsimilar threads of a retaining ring 86. The portion of the barrel 82which is external to the housing 5| bears a flange 86 abutting againstthe side wall thereof. To prevent light penetration of the housing 5! apacking ring 8? of felt or similar material is placed about the barrel82 on either side of the housing wall. Attached to the flange 88 is afilm chamber 83 for suitably disposing a strip of film 89 with respectto the lens 83. It is readily ap- 4 parent that a conventional cameramay be suitably adapted with the lens thereof extended through the wallof the housing 5| to accomplish the purposes of the foregoing.

Disposed between the mirror is and the lens 83 is an achromatic prismmounted on a pedestal 92 so that light reflected from the mirror isdirected to the lens 83 to be focused on the film strip 39. A projectoras is mounted on the same side wall of the housing 5! having theaperture BI and is provided with a bulb 9?, a lens 98, and a grid work99 disposed therebetween, the latter comprising a plurality of fineparallel mounted elements spaced apart by a predetermined distance toform a scale when projected. To direct the scale projected through thelens 93 onto the film 89 a mirror Hill is angularly mounted on thepedestal 92 between the lens 98 and the lens 83.

Since the apparatus described above is to be used for an accuratemeasurement of a beam H I of charged particles such as protons, thesource of such beam is known and the energy thereof determinable to liewithin a relatively small range. By using the intermediate value ofenergy of such range it is possible to calculate, by the use ofwellknown formulae, the half angle of the conical Cerenkov radiation tobe expected when the beam lii passes through a medium having a knownindex of refraction. For best result it is then necessary to set theposition of the mirror '15 at an angle equal to the determined angle ofthe Cerenkov radiation plus ninety degrees with respect to the centerline of the beam 1 l l, in which instance the Cerenkov radiation will besubstantially normal to the plane of the mirror and at the center of themeasurable range.

With the apparatus of the present invention placed along the path of thebeam I l i so that the beam passes through the hollow trunnions 28 and29, the mirror 16 is angularly disposed, substantially at the previouslydiscussed angle with re spect to the beam, with the coated surface awayfrom the source. Preferably the coating or refleeting surface of themirror i6 is substantially permeable to the beam and may be aluminum.A11 accurate determination may be made, even' though the direction ofthe beam 1 l I has not been absolutely established as parallel withrespect to a line extended between the centers of the trunnions 28 and2%, by taking two exposures. The second of such exposures should be madeafter inverting the equipment by means of the square plate 36. The twoexposures may then be averaged to determine the exact exposure whichwould have occurred had the axis of inversion of the equipment beenparallel to the pea-mill.

The angle between the axis of inversion and the normal to the aluminizedsurface of the mirror it may be accurately set by the use of a precisionangle template 25, as shown in Fig. 1, an adjustable bumper l2? and afixed circular bumper :23 mounted on the upper surface of the base plate22, and a pair of spaced-apart fixed circular bumpers 35 and #32 mountedon theunder surface of the housing 5i adjacent to the base plate. Theadjustable bumper i2? is provided th a, blo k 538 mounted on the baseplate as by screws E37, a screw H38 extended loosely through an apertureH9 in the block which is parallel to the base plate, a spring i i-ldisposed between the screw head and the block, a semi circular head hi2engaged by thev extendedend of the spring-mounted screw, and a pair ofad justment screws l 43 and l 44 threadably extended through the blockwith one on each side of the spring-mounted screw and parallel theretoto abut against the head. Thus by manipulation of the screws M3 and Hi lthe position of the bumper head M2 is adjustable.

The two fixed bumpers l3! and 132 on the housing iii are disposed to liealong a line parallel to the center line of the beam passageway throughthe housing. Preferably the precision angle template E25 is a righttriangle having one angle equal to one-half of the above-discussedhalf-angle of the anticipated Cerenkov radiation. Now, with the template26 placed on the plate 22 between the bumpers :21, HI, and I32, thefourth bumper 128 is disposed so that the template maintains the housing5| in a normal position such that the passage between the windows 58 isaligned with that between the trunnions 2i] and 29.

With the bumpers I2l, I28, l3I, and I32 disposed in the manner set forthabove, the template I25 is wedged therebetween to rotate the housing 53about the axis of the pivots 52 and 53. In such rotated position thenormal to the aluminized surface of the mirror 78 should lie parallel tothe axis of inversion of the housing 5|. Such condition be checked byviewing the image of a distant light source as reflected by thealuminiaed su face of the mirror I6 with a telescope equipped with apair of cross hairs. If the proper condition exists the position of theimage will not change when the instrument is inverted and any variationbe corrected by changes of the adjustable bumpers I21. Then by wedgingthe template E26 between the bumpers I21, I28, I3I, and 32 from theother direction, the angle between the normal to the mirror I6 and theaxis of inversion is eiiactly twice the angle of the template. For thepurpose of making the foregoing determinations a removable door I5I isprovided in a side wall of the housing 5| so that the aluminized surfaceof the mirror 'IB is visible when the housing is angularly disposed.

After aligning the equipment with respect to the beam i I i in theabove-discussed manner, the position of the film image may be related tothe angle between the ray directions inside the glass and the normal tothe aluminized surface. The foregoing may be accomplished by placing anassembly (not shown) carrying a cross hair arrangement in the planenormally occupied by the film strip 89. A source of light is then placedbehind the cross hair arrangement to project an image through the lens83, through the prism SI, and onto the mirror Iii. Such image is thenreflected along the same path to form a real image in the plane of thecros hair. With the prism set for minimum deviation and the cross hairset so that its reflected image coincides with itself, the projector 95is turned on and the scale 99 adiusted so that the central line thereofcoincides with the position of the cross hair and its image. Uponreassembly of the film chamber 88, accurate measurements of the energyof the beam I I I may be made from the exposed film strips 89, a sampleof which is illustrated in Fig. 4.

It will be readily apparent that the present invention may be used toaccurately determine the energy of beams of accelerated particles withinranges of such energy. By using various templates I25 having certainpredetermined angles, the apparatus may be aligned in the mannerdescribed in the foregoing.

While the salient features of the present invention have been describedwith respect to one embodiment, it will, of course, be apparent that nu=merous modifications may be made within. the spirit and scope of theinvention, and it is not desired to limit the invention to the exactdetails shown except insofar as they may be defined in the followingclaims.

What is claimed is:

1. In a device for measuring the energy of a charged particle beam, thecombination coznpris ing a supporting structure having two spacedapartand aligned bearing surfaces, a framework having two oppositely disposedhollow trunnions rotatably engaging said bearing surfaces, a squareplate centrally mounted on one of said trunnions and secured to saidframework, means associated with said plate for controlling the positionthereof in ninety degree steps, a housing impervious to light butpermeable to high energy particles, pivot means supporting said housingwithin said framework along the path between said trunnions, a mediumdisposed in said housing and in the path between said trunnions, andmeans disposed in said housing for registering the angle of lightradiation from said medium when high energy particles travel the pathbetween said trunnions as an indication of the energy of said particles.

2. In a device for accurately determining the energy of a beam ofcharged particles when the value of such energy is within apredetermined range, the combination comprising a supporting structurehaving two spaced-apart and aligned bearing surfaces, a framework havingtwo oppositely disposed hollow trunnions rotatably engaging said bearingsurfaces, a square plate centrally mounted on one of said trunnions andsecured to said framework, means associated with said plate forcontrolling the position thereof in ninety degree steps, a housingimpervious to light but permeable to high energy particles traveling thepath between said trunnions, pivot means supporting said housing withinsaid framework about an axis normal to and intersecting with the centerline of said trunnions, a mirror disposed in said housing and having acenter line thereof coincident with the axis of said pivot means, thereflecting surface of said mirror being permeable to said particles anddisposed away from the direction thereof, a strip of film disposedwithin said housing, and an optical system disposed between said mirrorand said film for directing light radiation developed within said mirroronto said film in accordance with the angle of said radiation withrespect to said particles as an indication of the energy thereof.

3. In a device for accurately determining the energy of a beam ofcharged particles when the value of such energy is within apredetermined range, the combination comprising a supporting structurehaving two spaced-apart and aligned bearing surfaces, a framework havingtwo oppositely disposed hollow trunnions rotatably engaging said bearingsurfaces, a square plate cen trally mounted on one of said trunnions andsecured to said framework, means associated with said plate forcontrolling the position thereof in ninety degree steps, a housingimpervious to light but permeable to high energy particles traveling thepath between said trunnions, pivot means supporting said housing withinsaid framework about an axis normal to and intersecting with the centerline of said trunnions, a glass mirror disposed in said housing andhaving a center line thereof coincident with the axis of said pivotmeans, the reflecting surface of said mirror being permeable to saidparticles and disposed angularly away from the direction thereof, theangle of said mirror with respect to the direction of said particlesbeing proportional to the energy corresponding to the median of saidpredetermined range, a strip of film disposed within said housing, andan optical system disposed between said mirror and said film fordirecting light radiation developed within said mirror onto said film inaccordance with the angle of said radiation with respect to saidparticles as an accurate indication of the energy thereof.

4. The combination of claim 1 wherein said medium is further defined asa sheet of glass having a light reflecting surface which is transparentto charged particles.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 2,401,327 Bednash June 4, 1946 2,405,063 Sisson July 30, 1946OTHER REFERENCES Cerenkov Radiation, Collins et al., Physical Rev., Oct.1, 1938, vol. 54, pp. 499-503.

